I don’t think we can make any direct suggestions about your findGrade method, but I strongly recommend you try it with a grade of 39 before you let it loose on an unsuspecting world examiner.
You have been given a specific signature for that method; why have you not used it?
Please have a look at our suggestions about formatting, because your formatting is inconsistent and may lose you marks.
Never use the \n character in String literals unless you specifically require it. Look here for what to use instead.
I think because it is assessed work, I am going to lock this thread to prevent anybody posting a full answer.